This invention relates to a process for producing a silicon carbide single crystal by allowing a vapor evaporated from a silicon raw material to react with a carbon material, and an apparatus for producing a silicon carbide single crystal. More particularly, it relates to a process for producing a silicon carbide single crystal by passing a vapor evaporated from a silicon raw material through a carbon member and bringing the vapor into contact with a seed crystal to thereby allow the silicon carbide single crystal to grow.
By the present invention, a high-quality and large-size silicon carbide single crystal can be produced continuously at a high rate under stabilized conditions.
Silicon carbide is a semiconductor material characterized as being thermally and chemically very stable and having a wide electronic energy band gap, and is used as a material for an environment-resistant element, a radiation-resistant element, a power element for controlling an electric power, and a short-wavelength light emitting element, which can be used even under high-temperature conditions.
A silicon carbide single crystal is usually produced by a sublimation technique (For example, see Japanese Patent Publication (Kouhyou) No. H3-501118). In the sublimation technique, a powdery silicon carbide raw material and a seed crystal are placed in a graphite crucible so as to confront each other and heated to a temperature of 1,800 to 2,400xc2x0 C. in an inert gas atmosphere. By the heating, the powdery silicon carbide raw material is decomposed and sublimed, and chemical species generated by sublimation reach the surface of the seed crystal maintained at a growth temperature where silicon carbide epitaxially grows as a single crystal. The composition of the sublimation vapor during the growth of a single crystal varies depending upon factors such as the decomposition and sublimation process of the silicon carbide crystal raw material, interaction among sublimation vapor components in the vapor phase, and contact reaction of silicon carbide with graphite on the inner wall of the reaction device. Proposals for suppressing the variation have been made which include, for example, a method of incorporating a silicon component or a carbon component in the powdery silicon carbide raw material, and a method of coating the inner wall of a crucible with tantalum.
To produce a silicon carbide single crystal having an enhanced crystallizability and homogeneity, a sublimation process using a powdery silicon carbide raw material with a high purity prepared by allowing silicon to react with carbon has been proposed (Japanese Unexamined Patent Publication No. H6-316499). 
The decomposition and sublimation vapor generated from a powdery silicon carbide contains Si, Si2C, SiC2 and others. As the amount of silicon components in the sublimation vapor is equimolar or more to the amount of carbon components, the composition of the powdery raw material gradually varies so that the amount of carbon increases. Thus, the partial pressure of the species contained in the sublimation vapor varies with time in the course of sublimation. The change in the composition of the sublimation vapor during the growth of the single crystal leads to reduction of crystallizability such as a crystal defect and a polymorphic mixing. A matter of importance is how to control these variation factors. In the process of crystallization into a silicon carbide crystal, various chemical species in the sublimation vapor naturally take different reaction routes. The reaction routes taken are considered to greatly depend on factors such as temperature of the raw material, variation of temperature distribution, reaction scheme of decomposition of silicon carbide raw material, and variation with time of the composition of raw material and others. These factors are difficult to control and therefore a silicon carbide single crystal having a high quality is difficult to prepare under stabilized conditions. The content of silicon in the vapor phase is liable to decrease in the course of growth of a single crystal as mentioned above, and thus, when the crystal growth is continued over a long period of time, deposition of a silicon carbide single crystal stops due to shortage of silicon in the vapor phase. Also, the composition of the powdery silicon carbide raw material varies so that the content of silicon decreases with time and finally the deposition of a silicon carbide single crystal stops. When the deposition of the single crystal stops, sublimation residue of the powdery silicon carbide raw material must be disposed. Therefore utilization efficiency of raw material in low and a continuous production is difficult to conduct. In addition, a powdery silicon carbide raw material having a high purity which is suitable for a single crystal with a high purity and a high quality used for semiconductors is difficult to prepare and is expensive.
A method of reacting silicon vapor with carbon to allow a silicon carbide single crystal to grow is known. As examples of such method, there can be mentioned (1) a method wherein silicon to used as a raw material, and silicon vapor evaporated from the silicon raw material within a graphite crucible is fed into a silicon carbide deposition chamber where the silicon vapor is brought into contact with carbon vapor generated from a graphite inner wall of the chamber whereby a silicon carbide single crystal grows (Japanese Examined Patent Publication No. S51-8400); (2) a method wherein silicon vapor as a raw material is brought into contact with a carbon plate to deposit a silicon carbide single crystal (U.S. Pat. No. 3,147,159); and (3) a method wherein silicon is used as a raw material, and silicon vapor evaporated therefrom within a graphite reaction tube is allowed to react with carbon vapor evaporated from the inner wall of the graphite reaction tube, to deposit a silicon carbide membrane on a silicon carbide single crystal plate placed within the graphite reaction tube (British Patent No. 1,031,783). 
In the methods described in Japanese Examined Patent Publication No. S51-8400 and U.S. Pat. No. 3,147,159, growth of silicon carbide crystals proceeds on naturally occurring nuclei of silicon carbide crystal on the graphite wall, and a multiplicity of single crystals in a leaf form are formed, and thus, a silicon carbide single crystal having a large size cannot be obtained.
In the method described In British Patent No. 1,031,783, the composition of the vapor phase can be maintained at constant, but, the contact area of silicon vapor with the inner wall of the reactive tube is small, and thus, the growth rate of silicon carbide single crystal is low, i.e., not higher than 0.3 mm/hr. Further, because the reactive tube itself is a raw material, the quantity of raw material is restricted and a single crystal having a large size cannot be produced. And, because continuous supply of a raw material cannot be affected in said method, it cannot be adopted for a process for producing a bulk single crystal.
In view of the problems of the foregoing prior art, an object of the present invention is to provide a process for producing a silicon carbide single crystal having a large size and high quality in a continuous manner, at a high rate and under stabilized conditions.
Another object is to provide an apparatus suitable for carrying out the above-mentioned production process.
To achieve the above-mentioned objects, the inventors have conducted extensive researches into a method for evaporating silicon to react with carbon to allow a silicon carbide single crystal to grow. As the results of the researches, it was found that a silicon carbide single crystal having high quality and a large size can be grown at a high rate by using high-purity silicon and carbon as raw materials, situating a seed crystal, making larger the contact area between silicon vapor and carbon and conducting the crystal growth under appropriate pressures. The present invention has been completed based on this finding.
Thus, in accordance with the present invention, there is provided a process for producing a silicon carbide single crystal wherein a silicon carbide single crystal in allowed to grow on a seed crystal substrate, characterized by allowing a vapor evaporated from a silicon raw material to pass through a heated carbon member and reach the seed crystal substrate.
Further, in accordance with the present invention, there is provided an apparatus for producing a silicon carbide single crystal comprising a reaction tube, a heating device and a graphite crucible situated in the reaction tube, characterized in that the lower part of the graphite crucible constitutes a silicon raw material-charging part; a seed crystal substrate is situated at the top of the graphite crucible; and a carbon member, through which a vapor evaporated from the silicon raw material is capable of passing, is disposed at an intermediate position between the silicon raw material-charging part and the seed crystal.